Thickness sensors are usually employed in SSTs or ATMs to determine whether the expected number of media sheets is present at a particular point during processing of the media and in particular to detect double or multiple picking where only one item of media should have been picked. In an ATM, the thickness of notes which go to make up the stack of notes which is presented to a user of the machine is checked during their transportation through the dispenser unit. If a reading is thicker than expected, it is assumed that the pick means has, for example, picked up two notes rather than one and the stack of notes is not presented to the user. Instead, the stack is diverted to a purge bin.
The use of a note thickness sensor in an ATM can be described with reference to FIG. 1 which shows a schematic side elevation view of a cash dispenser unit 73 of an ATM. The cash dispenser unit 73 holds a number of currency cassettes 89 each holding a stack of currency notes 68. Each cassette 89 holds only one denomination of currency notes, but different cassettes may contain different denominations. The notes 68 are held in place in the cassette by means of a pusher plate 69 which urges the stack of notes to one end of the cassette.
When one or more notes are to be dispensed from a particular cassette 89, the pick mechanism 74 associated with the cassette 89 draws a note from the cassette 89 such that its leading edge is gripped between drive rollers 90. The note is then fed along the feed path 72 by further drive rollers 92, through a retard mechanism 10 to a stacker wheel assembly 75. The stacker wheel assembly 75 comprises a hub 76 bearing tines 78 and is arranged to rotate on a shaft 77. As each note passes along the feed path 72, its thickness is checked by a compensated capacitance thickness sensor 118, in order to detect whether more than one note has been picked up inadvertently. In operation, the stacker wheel assembly 75, which comprises a hub 76 bearing tines 78 and arranged to rotate on a shaft 77. The stacker wheel assembly 75 rotates continuously in a counter-clockwise direction (for the arrangement shown in FIG. 1) and the note is fed into a compartment 81 formed between adjacent tines 78. If more than one note is to be dispensed, each note is fed into a successive compartment 81 as the stacker wheel assembly 75 rotates. Having completed half a rotation, the note is removed from the stacker wheel assembly 75 by fingers 94 of a stripper plate assembly 96 pivotally mounted on a shaft 98. Once removed from the stacker wheel, the note is placed on a belt 100 resting against the stripper plate assembly 96 and any subsequent notes which are to be dispensed simultaneously with the first note are placed on top of the first note to form a bundle 68′. When the required amount of notes (which may be just one note) have been assembled into the bundle 68′, a pair of belts 102 (only one of which is shown in FIG. 1) is rotated on a shaft 104 such that the bundle 68′ is trapped between the belts 100, 102. The bundle is then fed between belts 100, 102, 106, 108 through a note exit slot 110 in the housing 112 of the cash dispenser unit 73 usually to a position where the bundle 68′ can be collected by the user of the ATM.
If a multiple feeding is detected by the thickness sensor 118 or one or more of the notes are rejected for any reason, the bundle is not fed to the note exit slot 110. Instead the stripper plate assembly 96 is pivoted into a position as shown by the dashed outline 96′ and the belts 100, 102 are operated in the reverse direction to deposit the bundle 68′ into a reject note container 114 via an opening 116.
The compensated capacitance sensor 118 is neither as cheap nor as robust as is desirable. In normal use and in particular during maintenance and replacing media cassettes 89, sensors 118 tend to become inaccurate or become offset. This either results in multiple feeding not being detected or correctly assembled bundles being rejected. This means that the purge bin 114 becomes full and has to be emptied and the media cassettes 89 run out of notes more quickly than could otherwise be the case, requiring refilling. Faults require servicing the compensated capacitance sensor 118, which is a part of the machine which is not readily accessible.